1. Field of the Invention
The present invention relates to a sheet feeding apparatus which has sheets, such as a plurality of sheets of paper, stacked thereon, and supplies the sheets one by one to a main body of the device.
2. Description of the Related Art
Conventionally, in image forming apparatuses, such as a printer, a copying machine, and a facsimile, which perform separation and feeding for a plurality of stacked sheets and form an image on a sheet, there is a device which detects the amount of stacked sheets as described in Japanese Patent Application Laid-Open No. H03-079537. In this device, a message is issued or processing is changed depending on the amount of stacked sheets detected.
Additionally, in a sheet feeding device which has sheets stacked on a pressure plate, moves the pressure plate up and down during sheet feeding, and makes a feed roller abut the upper surface of a paper, thereby performing feeding, there is a device as described in Japanese Patent Application Laid-Open No. 2006-137564 for the purpose of preventing changes in the operating amount of the pressure plate depending on the stacked amount of sheets.
In the device disclosed in Japanese Patent Application Laid-Open No. 2006-137564, it is possible to keep constant the distance between a sheet upper surface and a feed roller during sheet separation irrespective of an amount of stacked sheets, and keep constant the operating amount of the pressure plate during sheet feeding, thereby keeping the timing of sheet conveyance constant. Also, since it is also possible to minimize the influence of the stacked sheets on alignment caused by the operation of the pressure plate, it is possible to perform stable sheet feeding.
However, when a dedicated switch, a dedicated sensor, and the like are provided for the detection of the amount of stacked sheets as described in Japanese Patent Application Laid-Open No. H03-079537, space and costs will increase mechanistically and electrically.
Additionally, in Japanese Patent Application Laid-Open No. H08-259039, a device which detects the stacked amount of sheets without a dedicated sensor by utilizing an existing sensor for detection of sheet conveyance is proposed.
However, since a change in the operating time of the pressure plate caused by the difference of the stacked amount is detected in Japanese Patent Application Laid-Open No. H03-079537, it is necessary for the operating time of the pressure plate to change according to an amount of stacked sheets.
Meanwhile, in the sheet feeding device in which the operating amount of the pressure plate is kept constant for stability of sheet feeding performance or shortening of time in Japanese Patent Application Laid-Open No. 2006-137564, the operating time of the pressure plate is constant irrespective of a stacked amount. Therefore, it is difficult to apply the technique disclosed in Japanese Patent Application Laid-Open No. H03-079537.